Microbiological analysis of fluid samples has important biological, medical, and industrial applications, for example, in clinics, in the food and beverage, pharmaceutical, personal care products, and environmental sectors. In general such analysis is aimed at determining the presence or absence of microorganisms in the sample, quantifying the amount of microorganisms present, and in some cases identifying an unknown microorganism to various levels of detail. Current standard methods of testing are often based on cell culturing, and take time to results of days to weeks depending on the type of sample and microorganism. There is a great need for microbiological analysis with increased throughput.
An example of such a rapid method is the one proposed by AES Chemunex (http://www.aeschemunex.com/). Their FDA-approved ScanRDI-system performs the analysis by laser scanning cytometry of filtered products. The steps of this method are filtering the fluid sample, staining the possibly present microbiological contaminants with a fluorescent dye, optically scanning the surface of the filter with a large laser spot (5-10 μm) for detecting the possibly present microbiological contaminants, and imaging the areas surrounding the contaminants with a high-resolution (0.5 μm) microscope having an automated stage. Aspects of the technique have been described in EP 0 713 087 B1.
An improved filter technology is provided by fluXXion (http://www.fluxxion.com/). The technique is based on lithographically defined micro-sieves, which have a single well-defined pore size (down to 0.2 μm), are optically flat (which is advantageous from the point of view of the subsequent optical scanning steps and also results in reduced backscattering) and thin so as to offer a low flow resistance and hence a higher filtration throughput compared to conventional membrane filters made from porous materials such as cellulose, nylon, polyvinyl chloride, polysulfone, polycarbonate, and polyester.
Existing devices and methods generally employ filters with pre-defined dimensions and only optimize the parameters of the filtering process, such as the flow-through speed, and of the scanning process, such as scanning speed and beam diameter. For example, the ScanRDI-system of AES Chemunex uses standard 25 mm membrane filters, usually polyester ChemFilters CB04 with a pore size of 0.45 μm.
It is an object of the invention to provide a particularly fast method and a particularly fast apparatus for filtering a given amount of a fluid and subsequently scanning the filter. In particular, it is an object of the invention to provide a method and apparatus that is faster than the prior art described above.
This object is achieved by the features of the independent claims. Further specifications and preferred embodiments are outlined in the dependent claims.